This research includes a range of projects that are linked by a common experimental approach, theoretical framework and general goal. The goal is to advance our understanding of the functional organization of the human visual system by tracing the flow of information through it. The theoretical framework is from systems engineering, with standard notions of nonlinearity, sampling, filtering and signal and noise taking an important role. The experimental approach is exclusively psychophysical, involving investigations of visual performance and visual phenomena. As in previous phases of the project, a central concern is the question: Why isn't vision perfect? Here we focus on the limits of visual sensitivity, to light and to contrast, attempting to relate the performance limits to what is known of the functional organization of the visual system in the retina and beyond. Much of the proposed work exploits visual nonlinearities as a surrogate microelectrode to reveal characteristics of the neural representation at intermediate stages of processing in the visual system. The study of rod-cone interaction in visual sensitivity is one example of this approach which we have exploited in the past, and we believe it will reward further investigation with the help of new physiological and theoretical perspectives. Another project elaborates on our finding that when a pattern alternates in contrast so rapidly that it is not perceived, it may nevertheless generate a pattern-specific, but not eye-specific sensitivity loss implying activation of binocular cortical neurons without perception. This means that intermediate stages of neural processing respond to visual input that is not consciously perceived, and supports our previous evidence that spatial and temporal resolution losses are distributed, with progressively narrower passbands as signals penetrate the system. PUBLIC HEALTH RELEVANCE This research will clarify at which stages of visual processing essential information is lost, and show whether information, for instance from the flicker of computer monitors, can reach the brain without reaching conscious awareness. The results will also help define the intensities necessary for visual performance at low light levels.